Ablation probe for delivering fluid through porous structure

ABSTRACT

Ablation probes are provided for perfusing the tissue, while the tissue is ablated. The ablation probe comprises an elongated shaft and an ablative element, such as a needle electrode. The ablation probe further comprises a lumen that extends through the probe shaft, which will be used to deliver a fluid to the distal end of the probe shaft for perfusion into the surrounding tissue. The ablation probe further comprises a porous structure that is associated with the distal end of the shaft in fluid communication with the lumen. For example, the distal end of the shaft, or the entirety of the shaft, can be composed of the porous structure. Or, if the ablative element is an electrode, the electrode can be composed of the porous structure. Because the pores within the porous structure are pervasive, the fluid will freely flow out into the tissue notwithstanding that some of the pores may become clogged.

FIELD OF THE INVENTION

The field of the invention relates generally to the structure and use ofradio frequency (RF) electrosurgical probes for the treatment of tissue.

BACKGROUND OF THE INVENTION

The delivery of radio frequency (RF) energy to target regions withinsolid tissue is known for a variety of purposes of particular interestto the present invention. In one particular application, RF energy maybe delivered to diseased regions (e.g., tumors) for the purpose ofablating predictable volumes of tissue with minimal patient trauma. RFablation of tumors is currently performed using one of two coretechnologies.

The first technology uses a single needle electrode, which when attachedto a RF generator, emits RF energy from the exposed, uninsulated portionof the electrode. This energy translates into ion agitation, which isconverted into heat and induces cellular death via coagulation necrosis.In theory, RF ablation can be used to sculpt precisely the volume ofnecrosis to match the extent of the tumor. By varying the power outputand the type of electrical waveform, it is possible to control theextent of heating, and thus, the resulting ablation. The secondtechnology utilizes multiple needle electrodes, which have been designedfor the treatment and necrosis of tumors in the liver and other solidtissues. In general, a multiple electrode array creates a larger lesionthan that created by a single needle electrode.

The size of tissue coagulation created from a single electrode, and to alesser extent a multiple electrode array, has been limited by heatdispersion. As a result, multiple probe insertions must typically beperformed in order to ablate the entire tumor. This process considerablyincreases treatment duration and patient discomfort and, due in largepart to the limited echogenicity of the ablation probe when viewed underultrasonography, requires significant skill for meticulous precision ofprobe placement. In response to this, the marketplace has attempted tocreate larger lesions with a single probe insertion. Increasinggenerator output, however, has been generally unsuccessful forincreasing lesion diameter, because an increased wattage is associatedwith a local increase of temperature to more than 100° C., which inducestissue vaporization and charring. This then increases local tissueimpedance, limiting RF deposition, and therefore heat diffusion andassociated coagulation necrosis.

It has been shown that the introduction of saline into targeted tissueincreases the tissue conductivity, thereby creating a larger lesionsize. Currently, this is accomplished by treating the tissue with aseparate syringe. See, e.g., Ahmed, et al., Improved Coagulation withSaline Solution Pretreatment during Radiofrequency Tumor Ablation in aCanine Model, J Vasc Interv Radio 2002, July 2002, pp. 717-724; Boehm,et al., Radio-frequency Tumor Ablation: Internally Cooled ElectrodeVersus Saline-enhanced Technique in an Aggressive Rabbit Tumor Model,Radiology, March 2002, pp. 805-813; and Goldberg et al., Saline-EnhancedRadio-Frequency Tissue Ablation in the Treatment of Liver Metastases,Radiology, January 1997, pp. 205-210. Treating the tissue with aseparate syringe, however, is not the most efficient and least invasivemanner to deliver saline to the target tissue, since it requires anadditional needle insertion and does not anticipate the tissue locationswhere the ablations will ultimately be performed.

It has also been shown that, during an ablation procedure, a needleelectrode can be used to perfuse saline (whether actively cooled or not)in order to reduce the local temperature of the tissue, therebyminimizing tissue vaporization and charring. A needle, however,typically cannot deliver the amount of saline necessary to significantlyincrease the conductivity of the tissue, either due to an insufficientnumber or size of perfusion openings within the needle electrode and/orthe occurrence of clogged openings resulting from the entrapment oftissue during introduction of the probe.

Thus, there is a need for an improved ablation probe that can maximizethe delivery of fluid to tissue in order to provide a more efficient,effective, and dynamic ablation treatment of tissue.

SUMMARY OF THE INVENTION

In accordance with the present inventions, an ablation probe isprovided. The ablation probe comprises an elongated shaft, which in thepreferred embodiment, is rigid, so that it can be percutaneously orlaparoscopically introduced into a patient's body. Alternatively, theprobe shaft can be flexible, e.g., if the ablation probe takes the formof an intravascular or extravascular catheter. The ablation probefurther comprises an ablative element Although many types of ablativeelements may be contemplated by the present invention, the ablativeelement preferably takes the form of electrode(s), e.g., a single needleelectrode or an array of electrodes. The ablation probe furthercomprises a lumen that extends through the probe shaft, which will beused to deliver a fluid to the distal end of the probe shaft forperfusion into the surrounding tissue.

The ablation probe further comprises a porous structure that isassociated with the distal end of the shaft in fluid communication withthe lumen. For example, the distal end of the shaft, or the entirety ofthe shaft, can be composed of the porous structure. Or, if the ablativeelement is an electrode, the electrode can be composed of the porousstructure. In this case, the porous structure is preferably composed ofan electrically conductive material, such as stainless steel, so that itis capable of conveying radio frequency (RF) energy. In this manner afluid can be conveyed through the lumen, and out through the porousstructure into adjacent tissue during the ablation process. In the casewhere the ablative element is a single needle electrode, the needleelectrode can be close ended, since perfusion of the fluid will occurthrough the porous structure. In general, a close ended needle electrodecan penetrate through tissue more accurately. Because the pores withinthe porous structure are pervasive, the fluid will freely flow out intothe tissue notwithstanding that some of the pores may become clogged.The porous structure may be macroporous or microporous, but in onepreferred embodiment, the effective diameter of the pores will fallwithin the range of 1-50 microns.

The ablation probe may optionally include a sleeve disposed around theshaft, e.g., to increase the effective shear strength of the probeand/or provide electrical insulation between tissue and the probe shaft.The ablation probe may also optionally include a connector assemblymounted to the proximal end of the shaft. In this case, the connectorassembly may have an perfusion inlet port in fluid communication withthe lumen for conveyance of the fluid.

In accordance with a first aspect of the present invention, the porousstructure has a porosity in the range of 20-80 percent, preferablywithin the range of 30-70 percent. In this manner, structural integrityof the shaft and/or ablative element(s) can be maintained, whileproviding a free flow of fluid through the many channels within theporous structure. In accordance with a second aspect of the presentinvention, the porous structure is microporous in order to increase thenumber of pores that will perfuse the fluid. In accordance with a thirdaspect of the present invention, the porous structure may haveinterconnecting pores that are arranged in a random manner, in order toprovide a more efficient flow of fluid through the porous structure.

In accordance with a fourth aspect of the present invention, a tissueablation system is provided. The system comprises an ablation probe,which may be, e.g., a surgical probe. The ablation probe comprises anablative element (e.g., such as those previously described) and aperfusion lumen. At least a portion of the ablation probe is composed ofa porous structure that is in fluid communication with the perfusionlumen. The porous structure can have the same structure and function asthe previously described porous structures. The system further comprisesen ablation source operably coupled to the ablative element. If theablative element is in electrode, the ablation source can be a RFgenerator. The system further comprises a fluid source operably coupledto the perfusion lumen. The system may optionally comprise a pumpassembly for pumping the fluid from the source through the perfusionlumen of the ablation probe.

In accordance with a fifth aspect of the present invention, a method ofassembling an ablation probe is provided. The method comprises shaping amass of particles (e.g., an electrically conductive powder) into anelongated shaft, and sintering the shaped particles to form a porousstructure within the shaft. Preferably, the mass of particles arecompacted prior to sintering in order to control the porosity of theporous structure. The method further comprises forming a longitudinallumen within the shaft, which can be performed by forming the mass ofparticles into a hollow shaft. The method further comprises forming anablative element on the distal end of the shaft. If the ablative elementis a single needle electrode, it can be formed with the shaft during theparticle shaping and sintering process. The resulting ablation probe canhave the same features as the previously described ablation probe.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a plan view of a tissue ablation system constructed inaccordance with one preferred embodiment of the present inventions;

FIG. 2 is a side view of a preferred probe assembly used in the tissueablation system of FIG. 1;

FIG. 3 is a side view of a preferred ablation probe used in the probeassembly of FIG. 2;

FIG. 4 is a side view of an alternative ablation probe that can be usedin the probe assembly of FIG. 2;

FIG. 5 is a cross-sectional view of a portion of the ablation probe ofFIG. 3;

FIG. 6 is a close-up view taken along lines 6-6 in FIG. 5;

FIG. 7 is a perspective view of an alternative probe assembly that canbe used in the tissue ablation system of FIG. 1, wherein the probeassembly is in its retracted state;

FIG. 8 is a perspective view of the probe assembly of FIG. 7, whereinthe probe assembly is in its deployed state; and

FIGS. 9A-9C illustrate cross-sectional views of one preferred method ofusing the tissue ablation system of FIG. 1 to treat tissue.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a tissue ablation system 100 constructed inaccordance with a preferred embodiment of the present inventions. Thetissue ablation system 100 generally comprises a probe assembly 102configured for introduction into the body of a patient for ablativetreatment of target tissue; a radio frequency (RF) generator 104configured for supplying RF energy to the probe assembly 102 in acontrolled manner; and a pump assembly 106 configured for perfusingfluid, such as saline, out through the probe assembly 102, so that amore efficient and effective ablation treatment is effected.

Referring specifically now to FIG. 2, the probe assembly 102 generallycomprises an ablation probe 110 and a cannula 108 through which theablation probe 110 can be introduced. As will be described in furtherdetail below, the cannula 108 serves to deliver the active portion ofthe ablation probe 110 to the target tissue. The cannula 108 has aproximal end 112, a distal end 114, and a perfusion lumen 116 (shown inphantom) extending through the cannula 108 between the proximal end 112and the distal end 114. An open tapered point 118 is formed at thedistal end 114 of the cannula 108 in order to facilitate introduction ofthe cannula 108 through tissue. As will be described in further detailbelow, the cannula 108 may be rigid, semi-rigid, or flexible, dependingupon the designed means for introducing the cannula 108 to the targettissue. The cannula 108 is composed of a suitable material, such asplastic, metal or the like, and has a suitable length, typically in therange from 5 cm to 30 cm, preferably from 10 cm to 20 cm. If composed ofan electrically conductive material, the cannula 108 is preferablycovered with an insulative material. The cannula 108 has an outsidediameter consistent with its intended use, typically being from 1 mm to5 mm, usually from 1.3 mm to 4 mm. The cannula 108 has an inner diameterin the range from 0.7 mm to 4 mm, preferably from 1 mm to 3.5 mm.

Referring further to FIG. 3, the ablation probe 110 generally comprisesa shaft 120 having a proximal end 122 and a distal end 124, a singletissue penetrating needle electrode 126 formed at the end of the distalshaft end 124, and a lumen 128 (shown in phantom) longitudinallyextending through the length of the shaft 120. The shaft 120 comprises awall 130 that is preferably composed of an electrically conductivematerial, such as stainless steel, nickel-titanium alloy,nickel-chromium alloy, spring steel alloy, and the like. As will bedescribed in further detail below, the shaft wall 130 is composed ofporous structure 132, as well as the needle electrode 126, (shown inFIG. 6) that facilitates the introduction of a fluid into the tissueduring the ablation process.

The needle electrode 126 is designed to penetrate into tissue as it isadvanced to the target tissue site. In the illustrated embodiment, theneedle electrode 126 has a closed-ended point, thereby facilitatingintroduction of the needle electrode 126 through the tissue along astraight line. Alternatively, the needle electrode 126 can have anopen-ended tapered tip similar to the tip 118 of the cannula 108.Because the perfusion of fluid is provided through the porous structureof the shaft 120, however, the use of an open-ended tapered tip,otherwise used for perfusion, is not needed. In the illustratedembodiment, the needle electrode 126 has a circular cross-section, butmay also have a non-circular cross-section as well. Like the probe shaft120, the needle electrode 126 is composed of an electrically conductivematerial, such as stainless steel, nickel-titanium alloy,nickel-chromium alloy, spring steel alloy, and the like. In fact, theneedle electrode 126 is preferably formed as a unibody structure withthe shaft 120, as will now be described.

Referring to FIGS. 5 and 6, the porous structure 132 of the probe shaft120 will now be described. The porous structure 132 comprises aplurality of pores 134 that extend through the thickness of the shaftwall 130 between the perfusion lumen 128 and exterior of the shaft 120.In the illustrated embodiment, the pores 134 are interconnected in arandom interstitial arrangement in order to maximize the porosity of theshaft wall 130. The porous structure 132 may be microporous, in whichcase, the effective diameters of the pores 134 will be in the 0.05-20micron range, or the porous structure 132 may be macroporous, in whichcase, the effective diameters of the pores 134 will be in the 20-2000micron range. A preferred pore size will be in the 1-50 micron range.The porosity of the porous structure 132, as defined by the pore volumeover the total volume of the structure, is preferably in the 20-80percent range, and more preferably within the 30-70 percent range.Naturally, the higher the porosity, the more freely the fluid will flowthrough the probe wall 130. Thus, the designed porosity of the porousstructure 132 will ultimately depend on the desired flow of the fluid.Of course, the porous structure 132 should not be so porous as to undulysacrifice the structural integrity of the ablation probe 110.

Thus, it can be appreciated that the pervasiveness of the pores 134allows the fluid to freely flow from the perfusion lumen 128, throughthe thickness of the shaft wall 130, and out to the adjacent tissue.Significantly, this free flow of fluid will occur even if several of thepores 134 have been clogged with material, such as tissue. For purposesof ease in manufacturability, the entire length of the probe shaft 120,including the needle electrode 126, is composed of the porous structure132. Alternatively, only the portion of the shaft 120 that will beadjacent the ablation region (e.g., the distal shaft end 124 includingthe needle electrode 126) and/or the needle electrode 126 is composed ofthe porous structure 132.

In the preferred embodiment, the porous structure 132 is formed using asintering process, which involves compacting a plurality of particles(preferably, a blend of finely pulverized metal powers mixed withlubricants and/or alloying elements) into the shape of the probe shaft120, and then subjecting the blend to high temperatures.

When compacting the particles, a controlled amount of the mixed powderis automatically gravity-fed into a precision die and is compacted,usually at room temperature at pressures as low as 10 or as high as 60or more tons/inch² (138 to 827 MPa), depending on the desired porosityof the probe shaft 120. The compacted power will have the shape of thehollow probe shaft 120 once it is ejected from the die, and will besufficiently rigid to permit in-process handling and transport to asintering furnace. Other specialized compacting and alternative formingmethods can be used, such as powder forging, isostatic pressing,extrusion, injection molding, and spray forming.

During sintering, the unfinished probe shaft 120 is placed within acontrolled-atmosphere furnace, and is heated to below the melting pointof the base metal, held at the sintering temperature, and then cooled.The sintering transforms the compacted mechanical bonds between thepower particles to metallurgical bonds. The interstitial spaces betweenthe points of contact will be preserved as pores. The amount andcharacteristics of the porosity of the structure 132 can be controlledthrough powder characteristics, powder composition, and the compactionand sintering process.

Porous structures can be made by methods other than sintering. Forexample, pores may be introduced by mechanical perforation, by theintroduction of pore producing agents during a matrix forming process,or through various phase separate techniques. Also, the porous structuremay be composed of a ceramic porous material with a conductive coatingdeposited onto the surface, e.g., by using ion beam deposition orsputtering.

Referring back to FIG. 3, the ablation probe 110 further comprises aconnector assembly 136 mounted on the proximal shaft end 122. Theconnector assembly 136 comprises a hollow handle piece 138 formanipulation by a physician, an perfusion inlet port 140, such as a maleluer connector, and a RF port 142. The perfusion inlet port 140 is influid communication with the perfusion lumen 128 of the shaft 120, andthe RF port 142 is in electrical communication with the shaft wall 130,and thus the needle electrode 126. The connector assembly 136 is alsoprovided with a nut 144, which engages the threads (not shown) of thecannula 108 in order to integrate the probe assembly 102 once the needleelectrode 126 is properly located at the target ablation site. Theconnector assembly 136 can be composed of any suitable rigid material,such as, e.g., metal, plastic, or the like.

Because the shear strength of the shaft 120 may be reduced due to itsporous nature, the ablation probe 110 comprises an optional sleeve 146that is disposed around the shaft 120 to increase the strength of theablation probe 110 (shown in FIG. 4). Preferably, the sleeve 146 housesthe entire length of the shaft 120, with the exception of the needleelectrode 126, which should be exposed to allow perfusion into thetarget ablation site. The sleeve 146 can be formed around the shaft 120in any one of a variety of manners, e.g., by co-extruding it over theshaft 120. If the cannula 108 is not used, or if the cannula 108 iscomposed of an electrically conductive material, the sleeve 146 ispreferably composed of an electrically insulative material, such asplastic. In this manner, the RF energy conveyed through the shaft 120will be concentrated at the target ablation site adjacent the needleelectrode 126.

Referring back to FIG. 1, the RF generator 104 is electrically connectedto the RF port 142 of the connector assembly 136 via an RF cable 148,which as previously described, is indirectly electrically coupled to theneedle electrode 126 through the shaft 120. The RF generator 104 is aconventional RF power supply that operates at a frequency in the rangefrom 200 KHz to 1.25 MHz, with a conventional sinusoidal ornon-sinusoidal wave form. Such power supplies are available from manycommercial suppliers, such as Valleylab, Aspen, and Bovie. Most generalpurpose electrosurgical power supplies, however, operate at highervoltages and powers than would normally be necessary or suitable forvessel occlusion. Thus, such power supplies would usually be operated atthe lower ends of their voltage and power capabilities. More suitablepower supplies will be capable of supplying an ablation current at arelatively low voltage, typically below 150 V (peak-to-peak), usuallybeing from 50 V to 100 V. The power will usually be from 20W to 200W,usually having a sine wave form, although other wave forms would also beacceptable. Power supplies capable of operating within these ranges areavailable from commercial vendors, such as Boston Scientific Corporationof San Jose, Calif., who markets these power supplies under thetrademarks RF2000™ (100W) and RF3000™ (200W).

RF current is preferably delivered from the RF generator 104 to theneedle electrode 126 in a monopolar fashion, which means that currentwill pass from the needle electrode 126, which is configured toconcentrate the energy flux in order to have an injurious effect on thesurrounding tissue, and a dispersive electrode (not shown), which islocated remotely from the needle electrode 126 and has a sufficientlylarge area (typically 130 cm² for an adult), so that the current densityis low and non-injurious to surrounding tissue. In the illustratedembodiment, the dispersive electrode may be attached externally to thepatient, e.g., using a contact pad placed on the patient's flank.

The pump assembly 106 comprises a power head 150 and a syringe 152 thatis front-loaded on the power head 150 and is of a suitable size, e.g.,200 ml. The power head 150 and the syringe 152 are conventional and canbe of the type described in U.S. Pat. No. 5,279,569 and supplied byLiebel-Flarsheim Company of Cincinnati, Ohio. The pump assembly 106further comprises a source reservoir 154 for supplying the fluid to thesyringe 152. The fluid can be optionally cooled to provide theadditional beneficial effect of cooling the needle electrode 126 and thesurrounding tissue during the ablation process. The pump assembly 106further comprises a tube set 156 removably secured to an outlet 158 ofthe syringe 152. Specifically, a dual check valve 160 is provided withfirst and second legs 162 and 164, of which the first leg 162 serves asa liquid inlet connected by tubing 166 to the source reservoir 154. Thesecond leg 164 is an outlet leg and is connected by tubing 168 to theperfusion inlet port 140 on the connector assembly 136.

Thus, it can be appreciated that the pump assembly 106 can be operatedto periodically fill the syringe 152 with the fluid from the sourcereservoir 154 via the tubing 166, and convey the fluid from the syringe152, through the tubing 168, and into the perfusion inlet port 140 onthe connector assembly 136. The fluid is then conveyed through theperfusion lumen 128 of the shaft 120, and out through the needleelectrode 126. Notably, the sleeve 146 (shown in FIG. 4) prevents thefluid from perfusing out the porous structure 132 along the length ofthe shaft, thereby forcing all of the fluid to perfuse out of the porousstructure 132 along the exposed needle electrode 126.

Other types of pump assemblies are also available for pumping fluidthrough the probe shaft 120. For example, a saline bag can simply beconnected to the fluid inlet port 140 on the connector assembly 136 viatubing, and then raised above the patient a sufficient height to providethe head pressure necessary to convey the fluid through the shaft 120and out of the needle electrode 126. Alternatively, pumps can beconveniently incorporated within the connector assembly 136.

The pump assembly 106, along with the RF generator 104, can includecontrol circuitry to automate or semi-automate the cooled ablationprocess. Further details on the structure and operation of a controlledRF generator/pump assembly suitable for use with the tissue ablationsystem 100 are disclosed in U.S. Pat. No. 6,235,022, which is herebyfully and expressly incorporated herein by reference. A commercialembodiment of such an assembly is marketed as the Model 8004 RFgenerator and Pump System by Boston Scientific Corporation, located inSan Jose, Calif.

Referring now to FIGS. 7 and 8, an alternative embodiment of a probeassembly 202, which can be used in the tissue ablation system 100, willnow be described. The probe assembly 202 generally comprises anelongated cannula 208 and an inner probe 210 slidably disposed withinthe cannula 208. The cannula 208 has a proximal end 212, a distal end214, and a perfusion lumen (not shown) extending through the cannula 208between the proximal end 212 and the distal end 214. An open taperedpoint 218 is formed at the distal end 214 of the cannula 208 in order tofacilitate introduction of the cannula 208 through tissue. As with thecannula 108, the cannula 208 serves to deliver the active portion of theinner probe 210 to the target tissue, and may be rigid, semi-rigid, orflexible depending upon the designed means for introducing the cannula108 to the target tissue. The cannula 208 may have the same materialcomposition and dimensions as the cannula 108.

The inner probe 210 comprises a reciprocating shaft 220 having aproximal end 222 and a distal end 224 (shown in FIG. 8); a perfusionlumen (not shown) extending through the shaft 220 between the proximalend 222 and distal end 224; a cylindrical interface 228 mounted to thedistal end 224 of the shaft 220; and an array 230 of tissue penetratingneedle electrodes 232 mounted within the cylindrical interface 228. Theshaft 220 is slidably disposed within the perfusion lumen of the cannula208, such that longitudinal translation of the shaft 220 in a distaldirection 234 deploys the electrode array 230 from the distal end 214 ofthe cannula 208 (FIG. 8), and longitudinal translation of the shaft 218in aproxiinal direction 236 retracts the electrode array 226 230 intothe distal end 214 of the cannula 108 (FIG. 7).

Like the previously described probe 110, the inner probe 210 comprises aporous structure (not shown) that allows fluid to perfuse into thetarget ablation site. Preferably, each of the needle electrodes 232 iscomposed of the porous structure. Alternatively, or optionally, thecylindrical interface 228 or the probe shaft 220, itself, can becomposed of the porous structure. Thus, like the ablation probe 110,fluid may flow through the perfusion lumen of the shaft 220, and outthrough the porous structure of the needle electrodes 232, oralternatively the cylindrical interface 228 or shaft 220. In the casewhere the probe shaft 220 is composed of the porous structure, thecannula 208 will contain the fluid along the probe shaft 220, such thatall of the fluid perfuses out the distal shaft end 224.

The probe assembly 202 further comprises a connector assembly 238, whichincludes a connector sleeve 240 mounted to the proximal end 212 of thecannula 208 and a connector member 242 slidably engaged with the sleeve240 and mounted to the proximal end 222 of the shaft 220. The connectormember 242 comprises an perfusion inlet port 244 and an RF port 246 inwhich the proximal ends of the needle electrodes 230 (or alternatively,intermediate conductors) extending through the shaft 220 of the innerprobe 210 are coupled. The connector assembly 238 can be composed of anysuitable rigid material, such as, e.g., metal, plastic, or the like.

RF current can be delivered to the electrode array 230 in a monopolarfashion, as previously described above, or in a bipolar fashion, whichmeans that current will pass between “positive” and “negative”electrodes 232 within the array 230. In a bipolar arrangement, thepositive and negative needle electrodes 232 will be insulated from eachother in any regions where they would or could be in contact with eachother during the power delivery phase.

Further details regarding needle electrode array-type probe arrangementsare disclosed in U.S. Pat. No. 6,379,353, entitled “Apparatus and Methodfor Treating Tissue with Multiple Electrodes,” which is hereby expresslyincorporated herein by reference.

Having described the structure of the tissue ablation system 100, itsoperation in treating targeted tissue will now be described. Thetreatment region may be located anywhere in the body where hyperthermicexposure may be beneficial. Most commonly, the treatment region willcomprise a solid tumor within an organ of the body, such as the liver,kidney, pancreas, breast, prostrate (not accessed via the urethra), andthe like. The volume to be treated will depend on the size of the tumoror other lesion, typically having a total volume from 1 cm³ to 150 cm³,and often from 2 cm³ to 35 cm³ The peripheral dimensions of thetreatment region may be regular, e.g., spherical or ellipsoidal, butwill more usually be irregular. The treatment region may be identifiedusing conventional imaging techniques capable of elucidating a targettissue, e.g., tumor tissue, such as ultrasonic scanning, magneticresonance imaging (MRI), computer-assisted tomography (CAT),fluoroscopy, nuclear scanning (using radiolabeled tumor-specificprobes), and the like. Preferred is the use of high resolutionultrasound of the tumor or other lesion being treated, eitherintraoperatively or externally.

Referring now to FIGS. 9A-9C, the operation of the tissue ablationsystem 100 is described in treating a treatment region TR within tissueT located beneath the skin or an organ surface S of a patient. The probeassembly 102 is described in this operation, although the probe assembly202 can be alternatively used. The cannula 108 is first introducedthrough the tissue T, so that the distal end 114 of the cannula 108 islocated at the treatment region TR, as shown in FIG. 9A. This can beaccomplished using any one of a variety of techniques. In the preferredmethod, the cannula 108 and probe 110 are introduced to the treatmentregion TR percutaneously directly through the patient's skin or throughan open surgical incision. In this case, the sharpened tip 118 of thecannula 108 facilitates introduction to the treatment region TR. In suchcases, it is desirable that the cannula 108 or needle be sufficientlyrigid, i.e., have a sufficient column strength, so that it can beaccurately advanced through tissue T. In other cases, the cannula 108may be introduced using an internal stylet that is subsequentlyexchanged for the ablation probe 110. In this latter case, the cannula108 can be relatively flexible, since the initial column strength willbe provided by the stylet. More alternatively, a component or elementmay be provided for introducing the cannula 108 to the treatment regionTR. For example, a conventional sheath and sharpened obturator (stylet)assembly can be used to initially access the tissue T. The assembly canbe positioned under ultrasonic or other conventional imaging, with theobturator/stylet then removed to leave an access lumen through thesheath. The cannula 108 and probe 110 can then be introduced through thesheath lumen, so that the distal end 114 of the cannula 108 advancesfrom the sheath into the treatment region TR.

After the cannula 108 is properly placed, the probe shaft 120 isdistally advanced through the cannula 108 to deploy the needle electrode126 out from the distal end 114 of the cannula 108, as shown in FIG. 9B.Once the cannula 108 and probe 110 are properly positioned, the ablationprobe 110 and cannula 108 can then be integrated with each other bythreading the nut 144 around the threaded potion of the cannula proximalend 112. The RF generator 104 is then connected to the connectorassembly 136 via the RF port 142, and the pump assembly 106 is connectedto the connector assembly 136 via the fluid inlet port 140, and thenoperated to ablate the treatment region TR, while perfusing thetreatment region TR with fluid, as illustrated in FIG. 9C. As a result,lesion L will be created, which will eventually expand to include theentire treatment region TR.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

1. An ablation probe, comprising: an elongated shaft having a distalend; an ablative element disposed on the distal end of the shaft; alumen longitudinally extending within the shaft; and a porous structureextending along a substantial entirety of the shaft in fluidcommunication with the lumen, the porous structure having pores witheffective diameters in the range of 1-50 microns, the entirety of theporous structure being composed of a metallic substance.
 2. The ablationprobe of claim 1, wherein the porous structure has a porosity in therange of 30-70 percent.
 3. The ablation probe of claim 1, wherein theshaft is a rigid shaft.
 4. The ablation probe of claim 1, wherein theporous structure is electrically conductive.
 5. The ablation probe ofclaim 1, wherein the porous structure is composed of a metallicsubstance.
 6. The ablation probe of claim 1, wherein the porousstructure has a porosity in the range of 20-80 percent.
 7. The ablationprobe of claim 1, wherein the porous structure has interconnectingpores.
 8. The ablation probe of claim 1, wherein the entirety of theshaft is composed of the porous structure.
 9. The ablation probe ofclaim 1, wherein the ablative element comprises at least one electrode.10. The ablation probe of claim 1, further comprising a connectorassembly mounted to a proximal end of the shaft, wherein the connectorassembly comprises a port in fluid communication with the lumen.
 11. Theablation probe of claim 1, further comprising a handle mounted to aproximal end of the shaft.
 12. An ablation probe, comprising: anelongated shaft having a distal end; an ablative element disposed on thedistal end of the shaft; a lumen longitudinally extending within theshaft; and a microporous structure extending along a substantialentirety of the shaft in fluid communication with the lumen, theentirety of the microporous structure being composed of a metallicsubstance.
 13. The ablation probe of claim 12, wherein the shaft is arigid shaft.
 14. The ablation probe of claim 12, wherein the microporousstructure is electrically conductive.
 15. The ablation probe of claim12, wherein the microporous structure has interconnecting pores.
 16. Theablation probe of claim 12, wherein the ablative element is composed ofthe microporous structure.
 17. The ablation probe of claim 12, whereinthe ablative element comprises at least one electrode.
 18. The ablationprobe of claim 12, wherein the microporous structure is composed of ametallic substance.
 19. The ablation probe of claim 12, furthercomprising a connector assembly mounted to a proximal end of the shaft,wherein the connector assembly comprises a port in fluid communicationwith the lumen.
 20. The ablation probe of claim 1, wherein the entiretyof the shaft is composed of the microporous structure.
 21. The ablationprobe of claim 12, further comprising a handle mounted to a proximal endof the shaft.
 22. An ablation probe, comprising: an elongated shafthaving a distal end; an ablative element disposed on the distal end ofthe shaft; a lumen longitudinally extending within the shaft; and aporous structure extending along a substantial entirety of the shaft influid communication with the lumen, the porous structure havinginterconnecting pores, the entirety of the porous structure beingcomposed of a metallic substance.
 23. The ablation probe of claim 22,wherein the pores are interconnected in a random arrangement.
 24. Theablation probe of claim 22, wherein the shaft is a rigid shaft.
 25. Theablation probe of claim 22, wherein the porous structure is electricallyconductive.
 26. The ablation probe of claim 22, wherein the ablativeelement is composed of the porous structure.
 27. The ablation probe ofclaim 22, wherein the ablative element comprises at least one electrode.28. The ablation probe of claim 22, further comprising a connectorassembly mounted to a proximal end of the shaft, wherein the connectorassembly comprises a port in fluid communication with the lumen.
 29. Theablation probe of claim 22, wherein the entirety of the shaft iscomposed of the porous structure.
 30. The ablation probe of claim 22,further comprising a handle mounted to a proximal end of the shaft. 31.A tissue ablation system, comprising: an ablation probe having anablative element and a perfusion lumen, substantially the entire lengthof the ablation probe being composed of a porous structure in fluidcommunication with the perfusion lumen, the porous structure havingpores with effective diameters in the range of 1-50 microns, theentirety of the porous structure being composed of a metallic substance;an ablation source operably coupled to the ablative element; and a fluidsource operably coupled to the perfusion lumen.
 32. The tissue ablationsystem of claim 31, wherein the porous structure has a porosity in therange of 30-70 percent.
 33. The tissue ablation system of claim 31,wherein the porous structure is electrically conductive.
 34. The tissueablation system of claim 31, wherein the porous structure hasinterconnecting pores.
 35. The tissue ablation system of claim 31,wherein the ablation probe is a rigid probe.
 36. The tissue ablationsystem of claim 31, wherein the ablative element comprises at least oneelectrode.
 37. The tissue ablation system of claim 31, wherein theablation source is a radio frequency (RF) ablation source.
 38. Thetissue ablation system of claim 31, further comprising a pump assemblyfor pumping fluid from the fluid source and through the perfusion lumenof the ablation probe.
 39. The tissue ablation system of claim 31,wherein the ablation probe comprises a shaft, the entirety of which iscomposed of the porous structure.
 40. The tissue ablation system ofclaim 31, further comprising a handle mounted to a proximal end of theshaft.
 41. A tissue ablation system, comprising: an ablation probehaving an ablative element and a perfusion lumen, substantially theentire length of the ablation probe being composed of a microporousstructure in fluid communication with the perfusion lumen, the entiretyof the micrororous structure being composed of a metallic substance; anablation source operably coupled to the ablative element; and a fluidsource operably coupled to the perfusion lumen.
 42. The tissue ablationsystem of claim 41, wherein the porous structure has interconnectingpores.
 43. The tissue ablation system of claim 41, wherein the ablationprobe is a rigid probe.
 44. The tissue ablation system of claim 41,wherein the ablative element comprises at least one electrode.
 45. Thetissue ablation system of claim 41, wherein the ablation source is aradio frequency (RF) ablation source.
 46. The tissue ablation system ofclaim 41, further comprising a pump assembly for pumping fluid from thefluid source and through the perfusion lumen of the ablation probe. 47.The tissue ablation system of claim 41, wherein the ablation probecomprises a shaft, the entirety of which is composed of the microporousstructure.
 48. The tissue ablation system of claim 41, furthercomprising a handle mounted to a proximal end of the shaft.
 49. A tissueablation system, comprising: an ablation probe having an ablativeelement and a perfusion lumen, substantially the entire length of theablation probe being composed of a porous structure in fluidcommunication with the perfusion lumen, the porous structure havinginterconnecting pores, the entirety of the porous structure beingcomposed of a metallic substance; an ablation source operably coupled tothe ablative element; and a fluid source operably coupled to theperfusion lumen.
 50. The tissue ablation system of claim 49, wherein thepores are interconnecting in a random arrangement.
 51. The tissueablation system of claim 49, wherein the ablation probe is a rigidprobe.
 52. The tissue ablation system of claim 49, wherein the ablativeelement comprises at least one electrode.
 53. The tissue ablation systemof claim 49, wherein the ablation source is a radio frequency (RF)ablation source.
 54. The tissue ablation system of claim 49, furthercomprising a pump assembly for pumping fluid from the fluid source andthrough the perfusion lumen of the ablation probe.
 55. The tissueablation system of claim 49, wherein the ablation probe comprises ashaft, the entirety of which is composed of the porous structure. 56.The tissue ablation system of claim 49, further comprising a handlemounted to a proximal end of the shaft.